The present invention relates to the x-ray tube art. It finds particular application in conjunction with metal insert frame x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with conventional x-ray diagnostic systems and other penetrating radiation systems for medical and non-medical examinations.
Typically, a high power x-ray tube includes an evacuated envelope made of metal or glass, which holds a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e., thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode and anode, which is also located within the evacuated envelope. This potential causes the electrons to flow from the cathode to the anode through the evacuated region within the interior of the evacuated envelope. A cathode focusing cup, which houses the cathode filament, focuses the electrons onto a focal spot on the anode. The electron beam impinges the anode with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through an x-ray transmissive window of the envelope to a beam limiting device or collimator, which is attached to an x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination, thereby allowing images of the patient or subject to be reconstructed.
During the production of x-rays, many electrons from the electron beam striking the anode are reflected from the anode and strike other regions of the x-ray tube. The reflected electrons are often referred to as secondary electrons, and the act of such reflected electrons striking other regions of the x-ray tube is often referred to as secondary electron bombardment. Accordingly, the temperature of the x-ray transmissive window or insert window rises rapidly once the anode power is applied. The rise in window temperature is caused by both the thermal radiation from the anode inside the insert frame and the secondary electron bombardment. Excessive window temperatures may destroy the window braze joints due to thermal stress caused by expansion differences between the window and the insert frame at operating temperatures.
Due to its excellent ability to withstand high voltage, oil is the preferred cooling fluid for an x-ray tube. The oil is circulated between the housing and the x-ray tube passing directly across the window of the x-ray tube. However, for a metal frame x-ray tube, the insert window receives extensive heat flux and oil cooling may not be sufficient. As a result, oil may boil locally at the insert window, depositing a layer of carbon on the window surface. Carbonization of the x-ray window significantly reduces window cooling and also deteriorates x-ray image quality due to x-ray absorption in the carbon layer.
One prior method for protecting the window from overheating includes a window heat shield, which shields a junction between the window and a metal envelope from secondary electron bombardment. Unfortunately, the heat shield method requires a heat shield material having properties of both high thermal conductivity and excellent x-ray transparency. Materials with these properties may be costly and not easily obtainable. Another prior device for protecting the x-ray window from overheating includes the use of a refrigeration cooled window joint. This method requires that a refrigeration system be attached to the x-ray tube. While this solution may serve to cool the window braze, it may be ineffective for solving the problem of oil carbonization at the center of the window. Another prior method for protecting the x-ray window from overheating includes the use of an electrode window, which is intended to deflect the back-scattered electrons, i.e., the secondary electron bombardment, from the window, therefore reducing window heat flux. However, an effective design of such a window is still unavailable.
The present invention contemplates a new and improved x-ray tube assembly having a liquid-free x-ray insert window, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, an x-ray tube assembly includes an x-ray tube housing, a cathode assembly, and a rotating anode assembly. An insert frame, which is supported within the x-ray tube housing, defines a substantially evacuated envelope in which the cathode and anode assemblies operate to produce x-rays. A dielectric liquid coolant flows between the x-ray tube housing and the insert frame. An x-ray transmissive window assembly extends between and in a fluid-tight relationship with the x-ray tube housing and the insert frame.
In accordance with a more limited aspect of the present invention, the x-ray transmissive window assembly includes an x-ray transmissive insert window, which is hermetically connected to the insert frame. An x-ray transmissive top plate is connected to and substantially surrounded by a flange, which is fastened to the x-ray tube housing. An annular side plate has a first end hermetically connected to at least one of the insert frame and insert window, and a second end which is connected to a bottom surface of the flange.
In accordance with a more limited aspect of the present invention, the annular side plate includes an inner surface and an extended outer surface having a plurality of fins in contact with the dielectric liquid coolant.
In accordance with another aspect of the present invention, an x-ray tube assembly includes a housing, a cathode assembly, and an anode assembly. A metal insert frame, which is disposed within the housing, defines an evacuated envelope in which the cathode and anode assemblies operate to produce x-rays. A cooling system circulates a dielectric liquid coolant between the housing and the metal insert frame. An x-ray transmissive window, through which x-rays produced by the cathode and anode assemblies pass, is brazed to the insert frame at a braze joint in a vacuum-tight manner. A cooling assembly, which is in thermal contact with the x-ray transmissive window at the braze joint, removes heat from the window without liquid coolant passing over the window.
In accordance with another aspect of the present invention, an x-ray tube assembly includes an outer housing, a metal insert frame supported within the outer housing, which defines an evacuated envelope in which cathode an anode assemblies operate to produce x-rays, and an x-ray transmissive insert window brazed to the insert frame at a braze joint. A method of cooling the x-ray transmissive insert window, in which no liquid coolant contacts the x-ray transmissive window, includes forming a fluid-free cooling chamber around and above the x-ray transmissive window and circulating a cooling fluid around the fluid-free cooling chamber.
One advantage of the present invention is that it relieves overheating of the window joint.
Another advantage of the present invention resides in increased image intensity.
Another advantage of the present invention resides in the elimination of oil carbonization on the x-ray window.
Yet another advantage of the present invention resides in reduced input power to achieve a selected x-ray output.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.